Tracer doses of Na24 and Cl36 are given intravenously in rhesus monkeys. Later, a tracer dose of Na22 is given. Samples of anterior and posterior chamber aqueous humor are obtained still later from one and then from the other eye. The concentrations of these isotopes is determined by appropriate counting methods. The method yields concentrations of ions at 4 different times in the same animal under physiologic conditions. The experiments, repeated at different time intervals with a number of animals gives the time course of entrance of chloride and sodium ions into the anterior and posterior aqueous humor. Analysis of the data will help determine if each of the ions enters by a diffusional or by a unidirectional transport process.